In solar cell production for the photovoltaic, semiconductor and microelectronics industries the main technique used for slicing of silicon blocks into wafers is multi wire sawing. This process results in high throughput and gives an excellent surface quality. In recent times the industry has been moving towards the recycling of the abrasive slurry used in wire saw cutting, in order to improve efficiency and reduce waste. This in turn has been increasing the demand for tools to monitor the quality of the slurry during the process.

Slicing silicon wafers

Silicon blocks are sliced into wafers using a wire saw, consisting of a single wire with a typical diameter of 80 to 200 µm. This is fed from a supply spool through a wire tensioning system, and is wound around guide rollers to form a wire web, as shown in in figure 1. Finally the wire is collected on a take up spool. As the wire is wound from one side to the other it passes into the sawing channel where the abrasive slurry is supplied and is then carried on the wire in order to perform the cut-grinding process. As silicon work pieces are pushed against the wire web they are sliced into thousands of wafers in a single run.

The abrasive slurry is a suspension of hard grinding particles, such as silicon carbide, in glycol or oil. The abrasive materials are manufactured under strict quality control procedures so that they have the levels of precision required for the process. Traditionally, after the cutting process the exhausted slurries are discarded. The slurry becomes exhausted when there is a high proportion of fine silicon powder particles and broken abrasive particles present. Additionally the heat and chemical reactions during the process can generate particle aggregates. Both of these factors reduce the efficiency of the process and the quality of the wafers produced.

However, the industry has recently pushed towards recycling of the abrasive slurry in order to reduce not only production costs but also the amount of industrial waste. One method of recycling is to remove the solid material from the liquid by centrifuge. The reusable abrasive material is then separated from fine material and is reintroduced into the liquid.

Monitoring the recycling process

The technique of Flow Particle Image Analysis (using the Sysmex FPIA-3000 system from Malvern Instruments) is being used to monitor the deterioration of abrasive slurries in order to establish when a slurry should be withdrawn from production and recycled. Suspended sample is measured in real time and images of up to 360,000 particles are captured by a CCD camera for examination. Analysis, driven by standard operating procedures (SOP) takes around two minutes.

Such a measurement protocol also provides statistical results in terms of particle size and shape as well as together with particle count. This allows quantitative monitoring of the percentage of unwanted fine components in the slurry, such as worn grinder pieces and waste silicon powder. Additionally a scattergram that plots size versus shape of the particles allows distinction between primary particles and aggregates on the basis of differences in size and shape.

Analysis before and after the wire saw process

An abrasive slurry was analyzed before and after being used in the wire saw process. Figure 2 shows the results. The number-based Circular Equivalent Diameter size distributions show a large increase in the proportion of fine material present in the abrasive slurry sample after the wire saw process, compared with beforehand. As well as showing the change in the level of fine material (highlighted in green), the scattergrams of Circular Equivalent Diameter versus Circularity also indicate the presence of agglomerated particles (highlighted in blue) following the wire saw cutting process, which were not present initially. Regions of the scattergram can be defined in the SOP, allowing quantification and comparison of the proportions of unwanted particles and the usable grinder particles (highlighted in red). Additionally, images of every particle analyzed are retained by the system. Figure 2 shows example images of the different types of particles present in the abrasive slurry samples after the wire saw cutting process.

Conclusion

The analyses presented here show that the instrument under test can be used to monitor the deterioration of abrasive slurries in the wire saw cutting process for slicing silicon blocks into wafers. Quantitative analysis allows the presence of unwanted particles to be monitored. Therefore, decisions can be made as to when to remove slurry from production and send it for recycling. Additionally the image analysis system can be used to monitor the recycling process to ensure the unwanted particles are successfully removed.